Radiation image pickup apparatuses adopting digital flat panel detectors (hereinafter referred to as FPD detectors or simply detectors) formed by a semiconductor material are being put to practical use as image pickup apparatuses used for medical image diagnosis and non-destructive testing using X-rays. Such radiation imaging apparatuses are used, for example, as digital image pickup apparatuses for general still image capture such as radiography and moving image capture such as fluoroscopy in medical image diagnosis. Flat panel detectors can be classified into “indirect conversion” and “direct conversion” detectors differentiated by the radiation detection principle. A detector of indirect conversion type is known in which a conversion element obtained by combining a photoelectric conversion element using amorphous silicon and a wavelength conversion member that converts radiation into light in a wavelength band that can be detected by the photoelectric conversion element with each other is used. A detector of direct conversion type is also known in which a conversion element that directly converts radiation into charge using a material such as amorphous selenium is used.
In an image pickup apparatus, dangling bonds and defects in a semiconductor layer of each conversion element including the semiconductor layer composed of an amorphous semiconductor can function as trap centers for charge carriers. When an image is captured a plurality of times, charge generated by radiation or light radiated in a previous image pickup operation might be trapped by trap levels. In such a case, it is possible that a so-called lag, in which the charge trapped in the previous image pickup operation affects an image obtained in a subsequent image pickup operation, can be generated in the image obtained in the subsequent image pickup operation. Although the lag can be suppressed by increasing a time (hereinafter referred to as image pickup operation intervals) between the previous image pickup operation and the subsequent image pickup operation, usefulness of the imaging apparatus decreases when the image pickup operation intervals are increased. Therefore, the image pickup apparatus is required to decrease the image pickup operation intervals while suppressing a lag that can be generated in a subsequent image pickup operation.
In order to suppress the lag, reset operations have been disclosed, for example in patent literature (PTL) documents (PTL 1 and PTL 2) discussed below. Specifically, in order to suppress lag between subsequent imaging operations, a voltage different from a voltage during an image pickup operation is applied to each conversion element such as a photodiode or an MIS photoelectric conversion element between a plurality of image pickup operations. More specifically, in PTL 1, a reverse voltage larger than or a reverse voltage smaller than a reverse voltage of the photodiode during the image pickup operation or a forward voltage is applied to the photodiode during the reset operation. In PTL 2, a voltage different from the voltage during the image pickup operation is applied to each MIS photoelectric conversion element such that electrodes of the MIS photoelectric conversion element become ground during a sleep (reset) operation. In PTL 1 and PTL 2, it is disclosed that charge that can be a lag is removed from each conversion element by such reset operations.